Hydrocarbon resources such as petroleum or natural gas have come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation.
As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9,000 m worldwide and greater than 6,000 m in Japan.
In wells that are continuously excavated, the productive layer is stimulated in order to continuously excavate hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time and subterranean formations of which permeability is insufficient from the beginning. Known stimulation methods include acid treatment and fracturing methods (Patent Document 1).
Acid treatment is a method in which the permeability of the productive layer is increased by injecting a mixture of strong acids such as hydrochloric acid and hydrofluoric acid into the productive layer and dissolving the reaction components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out.
Thus, methods for forming cracks (fractures) in the productive layer using fluid pressure (also called “fracturing” or “hydraulic fracturing”) have received attention.
Hydraulic fracturing is a method in which fractures are generated in the productive layer by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical hole is drilled, and then the vertical hole is curved and a horizontal hole is drilled in a subterranean formation several thousand meters underground. Fracturing fluid is then fed into these boreholes (meaning holes provided for forming a well, also called “downholes”) at high pressure, and fractures are produced by the hydraulic pressure in the deep subterranean productive layer (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive layer is thereby stimulated in order to extract the hydrocarbon resource through the fractures.
The efficacy of hydraulic fracturing has also been examined for the development of unconventional resources such as shale oil (oil that matures in shale) and shale gas.
Fractures formed by fluid pressure such as hydraulic pressure immediately close due to formation pressure when the hydraulic pressure is no longer applied.
To prevent a fracture from closure, a proppant is included in the fracturing fluid (that is, the well treatment fluid used in fracturing), which is fed into the borehole, thereby distributing the proppant in the fracture.
Inorganic or organic materials are used as proppants included in fracturing fluid, but silica, alumina, and other inorganic particles have been conventionally used, and sand particles such as 20/40-mesh have been widely used because they are capable of preventing fracture closure in a very deep subterranean environment under high-temperature and high-pressure for a long time.
Various types of water-based fluids, oil-based fluids, and emulsions are used as well treatment fluids such as fracturing fluid.
Because the well treatment fluid needs to have the function of transporting the proppant to the location where the fracture is generated in the borehole, it generally needs to have a prescribed viscosity, good proppant dispersibility, ease of after-treatment, and low environmental load.
Furthermore, fracturing fluid sometimes contains a channelant in order to form flow paths through which shale oil, shale gas, and the like can pass among the proppant.
Accordingly, in addition to the proppant, various additives are used in well treatment fluid, such as channelants, gelling agents, antiscale agents, acids for dissolving rock and the like, friction-reducing agents, and the like.
The following method is typically used to produce fractures by hydraulic pressure in the productive layer of a deep subterranean formation (layer that produces the hydrocarbon resource such a petroleum such as shale oil or natural gas such as shale gas) using fracturing fluid.
Specifically, fracturing is performed by partially plugging a prescribed section of a borehole (downhole) drilled into a subterranean formation several thousand meters deep while isolating sequentially from the tip portion of the borehole, and feeding fracturing fluid at high pressure into the plugged section to produce fractures in the productive layer.
Then, the next prescribed section (typically ahead of the preceding section, i.e., a section closer to the ground surface) is plugged and fracturing is performed.
After that, this process is repeated until the required isolation and fracturing have been completed.
Stimulation of the productive layer is sometimes also performed again by fracturing not only for drilling of new wells but for desired sections of boreholes that have already been formed.
In this case as well, the operations of borehole plugging, fracturing, and the like are similarly repeated.
Additionally, there are also cases where, to perform completion of the well, the borehole is plugged to block fluid from below, and after finishing of the top portions thereof is performed, the plugging is released.
Various methods are known for plugging and fracturing of boreholes, and Patent Documents 2 and 3 disclose plugs that can plug or fix a borehole (also called a “frac plug,” “bridge plug,” “packer,” or the like).
Patent Document 2 discloses a downhole plug for well drilling (also called “plug for well drilling” or simply “plug” hereinafter), and specifically discloses a plug comprising a mandrel (main body) having a hollow part in the axial direction, a ring or annular member along the axial direction on the outer circumferential surface orthogonal to the axial direction of the mandrel, a first conical member and slip, a malleable element formed from elastomer, rubber, or the like, a second conical member and slip, and an anti-rotation feature.
Plugging of the borehole by this plug for well drilling is performed as follows.
Specifically, by moving the mandrel in the axial direction, as the gap between the ring or annular member and the anti-rotation feature gets smaller, the slip contacts the slanted face of the conical member and proceeds along the conical member, thereby moving so as to expand radially, and then the tip of the slip comes into contact with the inner wall of the borehole and is fixed in the borehole, and also, the malleable element deforms by diametric expansion as the distance in the axial direction shrinks, and comes into contact with the inner wall of the borehole to seal the borehole.
It is described that metal materials (aluminum, steel, stainless steel, and the like), fibers, wood, composite materials, plastics, and the like are widely exemplified as materials that form plugs, and that composite materials containing a reinforcing material such as carbon fibers, especially polymer composite materials of epoxy resin, phenol resin, and the like, are preferred, and that the mandrel is formed from aluminum or a composite material.
Plugs for well drilling are arranged sequentially inside a borehole until the well is completed, but at the stage when production of petroleum such as shale oil or natural gas such as shale gas (sometimes collectively called “petroleum and natural gas” hereinafter) is begun, the plugging of the borehole by the slip, which is a member of the plug for well drilling, and by the diameter-expandable annular seal member needs to be released and the plug needs to be removed.
Because the plug is typically not designed to be retrievable after use and release of plugging, it is removed by destruction or by making it into small fragments by fracturing, perforation, or another method, but substantial cost and time are required for fracturing, perforation, and the like.
There are also plugs specially designed to be retrievable after use (retrievable plugs), but since plugs are placed deep underground, substantial cost and time are required to retrieve all of them.
Patent Document 3 discloses a disposable downhole tool (meaning a downhole plug or the like) or a member thereof containing a biodegradable material that degrades when exposed to the environment inside a well, and as the biodegradable material, discloses a degradable polymer such as an aliphatic polyester such as polylactic acid.
Additionally, Patent Document 3 describes a combination of a tubular body element having an axial-direction flow bore, a packer element assembly formed of an upper sealing element, a center sealing element, and a lower sealing element along the axial direction on the outer circumferential surface orthogonal to the axial direction of the tubular body element, a slip, and a mechanical slip body.
Furthermore, Patent Document 3 discloses that fluid flow in only one direction is allowed due to the fact that a ball is set in the flow bore of the tubular body part.
However, Patent Document 3 does not disclose whether a material containing a biodegradable material is used for a downhole tool or any part thereof.
Based on increased demands such as securement of energy resources and environmental protection, and particularly due to the fact that excavation conditions have become more harsh and diverse, such as increased depth, while excavation of unconventional resources is expanding, for downhole tools such as plugs for well drilling there has been a demand for a seal member for downhole tools, a plug for well drilling, and a method for well drilling that can reliably seal a borehole to withstand the high fluid pressure of fracturing and the like, and that is easy to remove and makes it easy to secure a flow path, thereby reducing expense and shortening processes of well drilling.